A fuel-saving apparatus using a variable preload of vehicle bearings according to the present invention includes a housing that supports a driving shaft of a vehicle, a bearing that is mounted in the housing and rotatably supports the driving shaft, and a variable preload unit that is provided at the housing or a side adjacent to the housing and applies a preload to an outer wheel or an inner wheel of the bearing. The fuel-saving apparatus using such a variable preload may reduce a driving load acting on the driving shaft by a fixed preload acting by the bearing, thereby reducing the driving load of an engine and saving fuel.